The present invention relates to a spatial light modulation method and a spatial light modulation device that uses a spatial light modulator with liquid crystal as the light modulation material, and particularly to a spatial light modulation method and, a spatial light modulation device that uses a reflection type spatial light modulator.
There are two types of spatial light modulators: intensity modulation types and phase modulation types. Many spatial light modulators are the intensity modulation type, which are used in liquid crystal televisions, projector light bulbs, and the like. On the other hand, the phase modulation types show promise in fields such as light information processing and hologram processing. This is because the phase modulation types differ from intensity modulation types in that they have high light usage efficiency. A system that uses a phase modulating type spatial light modulator is disclosed by J. Gluckstad et al. in xe2x80x9cLossless Light Projectionxe2x80x9d, OPTICS LETTERS, Vol. 22, No. 18, 1997.
Phase modulation type spatial light modulators include reflection types and transmission types. Reflection type spatial light modulators differ from transmission types in that the same surface serves as the light input surface for the read light and the light output surface for the modulated light. For this reason, normally the modulated light is separated from the read light using a half mirror. However, as a result, there is a problem in that light usage efficiency drops. The benefit of using a phase modulation type is lost.
It is an objective of the present invention to take the above-described problem into account and provide a spatial light modulation method and a spatial light modulation device that uses a reflection type spatial light modulator and that has high light usage efficiency.
In order to overcome the above-described problem and other problems, the present invention provides a spatial light modulation device, comprising: a light source for outputting read light; and a reflection type spatial light modulator, the reflection type spatial light modulator including a light modulation layer having liquid crystal as light modulation material, a light reflection layer, a light input surface positioned at one side of the light modulating layer opposite from the light reflection surface, and voltage application means for applying an electric voltage to the light modulation layer, the reflection type spatial light modulator receiving the read light at the light input surface, transmitting the read light through the light modulation layer, reflecting the read light off the light reflection layer, and again transmitting the read light through the light modulation layer, thereby performing light modulation in the light modulation layer twice, and then outputting the modulated light from the input surface, wherein the light source and the reflection type spatial light modulator are arranged so that the read light falls incident on the input surface following an input optical axis that extends at a slant with respect to the light reflection layer and so that the read light outputs from the input surface following a reflection optical axis that extends at a slant with respect to the light reflection layer, wherein the read light includes approximately 100% P-polarized light component that has a polarization direction within a normal plane which is defined to include the input optical axis, the reflection optical axis, and a normal line that extends normal to the light reflection layer, and wherein the light modulation layer has liquid crystal molecules which are oriented so that they are arranged, without any spiral structure with respect to the normal line, within a plane approximately parallel with the normal plane and so that they are tilted, in association with application of the electric voltage by the voltage application means, within the plane which is approximately parallel with the normal plane, thereby causing no twist between the liquid crystal molecules and the oscillating plane of the P-polarized light component of the read light.
According to the spatial light modulation device of the present invention with this configuration, the read light falls incident on the input surface of the reflection type spatial light modulator at a slant. Accordingly, it is possible to separate, without using a half mirror, the read light inputted to the reflection type spatial light modulator from the read light reflected from the reflection type spatial light modulator. Accordingly, light usage efficiency can be increased and the freedom of arrangement of the input and output optical systems can be increased.
Additionally, according to the present invention, read light that includes approximately 100% P-polarized light component falls incident on the input surface at a slant. The P-polarized light component of the read light has a polarization direction within the normal plane that includes the normal line of the light reflection layer of the spatial light modulator and the input optical axis of the read light. Also, the liquid crystal in the light modulation layer is oriented so that they tilt, in association with application of voltage, within the plane that is substantially parallel with the normal plane. Accordingly, no twist develops between the P-polarized light component of the read light and the arrangement direction of the liquid crystal molecules. For this reason, only phase modulation is attained, without any rotation of the polarization plane.
Because the same is for the reflected light, the polarization plane of the modulated light that is finally outputted from the reflection type spatial light modulator has the same polarization plane with that of the input light. Accordingly, the modulated light includes approximately 100% P-polarized light component, and is outputted. It is possible to maintain a high diffraction efficiency.
In particular, it is desirable that the read light may include 100% P-polarized light component, and the light modulation layer may have the liquid crystal molecules which are oriented so that they are arranged, without any spiral structure with respect to the normal line, within a plane parallel with the normal plane and so that they are tilted, in association with application of the electric voltage by the voltage application means, within the plane which is parallel with the normal plane, thereby causing no twist between the liquid crystal molecules and the oscillating plane of the read light.
In this case, no twist occurs between the P-polarized read light and the arrangement direction of the liquid crystal molecules. Accordingly, phase-only modulation can be attained without any rotation of the polarization plane. Accordingly, an extremely high diffraction efficiency can be maintained.
It is desirable that the liquid crystal in the light modulation layer be processed in the homeotropic or homogeneous orientation. By processing the liquid crystal in the homeotropic or homogeneous orientation, the liquid crystal molecules are aligned within a predetermined plane with no spiral structure. By aligning this predetermined plane approximately with the normal plane, it is possible to perform phase modulation without any rotation of the polarization plane.
According to another aspect, the present invention provides a spatial light modulation method, comprising the steps of: preparing a reflection type spatial light modulator, the reflection type spatial light modulator including a light modulation layer having liquid crystal as light modulation material, a light reflection layer, a light input surface positioned at one side of the light modulating layer opposite from the light reflection surface, and voltage application means for applying an electric voltage to the light modulation layer, the reflection type spatial light modulator being for receiving read light at the light input surface, transmitting the read light through the light modulation layer, reflecting the read light off the light reflection layer, and again transmitting the read light through the light modulation layer, thereby performing light modulation in the light modulation layer twice, and then outputting the modulated light from the input surface; and inputting the read light to the reflection type spatial light modulator in a manner that the read light falls incident on the input surface following an input optical axis that extends at a slant with respect to the light reflection layer and that the read light outputs from the input surface following a reflection optical axis that extends at a slant with respect to the light reflection layer, wherein the read light includes approximately 100% P-polarized light component that has a polarization direction within a normal plane which is defined to include the input optical axis, the reflection optical axis, and a normal line that extends normal to the light reflection layer, and wherein the light modulation layer has liquid crystal molecules which are oriented so that they are arranged, without any spiral structure with respect to the normal line, within a plane approximately parallel with the normal plane and so that they are tilted, in association with application of the electric voltage by the voltage application means, within the plane which is approximately parallel with the normal plane, thereby causing no twist between the liquid crystal molecules and the oscillating plane of the P-polarized light component of the read light.
In particular, it is desirable the read light may include 100% P-polarized light component, and the light modulation layer may have the liquid crystal molecules which are oriented so that they are arranged, without any spiral structure with respect to the normal line, within a plane parallel with the normal plane and so that they are tilted, in association with application of the electric voltage by the voltage application means, within the plane which is parallel with the normal plane, thereby causing no twist between the liquid crystal molecules and the oscillating plane of the read light.
Also, it is desirable that the liquid crystal in the light modulation layer be processed in the homogeneous or homeotropic orientation.